In Summon Night: Twin Age, the player takes control of a female protagonist named Reiha along with a "summon beast" named Aldo. As the story is told, Reiha was a powerful magician as a child, but an accident sent her and Aldo to a island called Jarazi with a race of half-monster, half-human's and communicate with the Nature Spirits of the Island. One day the spirits start to act strange and begin attacking people so Reiha and Aldo start a quest to learn what has happened that will eventually take them back to Reiha's world.

The gameplay is typical-RPG style, moving around the map, talking to people and battling monsters. The battles consist of tapping the monsters or drawing lines to conjure spells on the bottom screen with the stylus pen to launch attacks on them. Destroy these monsters will earn the player experience points, letting them become more powerful as they have more people join their party.

The player can conjure monsters, collect weapons along with making their own and has multiple endings along with the option to play as either Reiha or Aldo. Players can also trade items with other players using DS wireless.